The cardiac Na+ channel SCN5A (Nav1.5) and the inward depolarizing Na+ current (INa) play a critical role in regulating the action potential of myocytes in the atrium, ventricle, and specialized conduction system. Mutations in SCN5A are associated with several arrhythmia phenotypes including long QT syndrome, Brugada syndrome, and dilated cardiomyopathy. Brugada Syndrome has also been linked to a mutation in the gene for the glycerol-3-phosphate dehydrogenase-like (GPD1-L) protein, which is dependent on the balance of cellular NADH and NAD+, and thus the energetic state of the cell. SIRT1 is a mammalian protein lysine deacetylase which belongs to the SIRTUIN family of NAD+-dependent deacetylases. SIRT1 targets many proteins for lysine deacetylation in response to changes in the energetic state of the cell. Whether SIRT1 targets cardiac ion channels, and SCN5A in particular, and thus plays a role in regulating cardiac excitability is not known. Moreover, potential interactions between GPD1-L, the metabolic state of ventricular myocytes, and SIRT1 activity are completely unexplored. This revised application is based on very novel preliminary data that SIRT1 interacts with Nav1.5 and GPD1-L to regulate INa. It hypothesizes that dynamic lysine acetylation and deacetylation is a previously undescribed post-translational modification of SCN5A that regulates INa, that SIRT1-dependent deacetylation of Nav1.5 increases INa, that GPD1-L mutations alter INa through changes in SIRT-mediated deacetylation of Nav1.5, and that changes in SIRT1 can modify arrhythmic risk. It will critically test these hypotheses using HEK 293 cells, rat neonatal cardiac myocytes, iPS-derived cardiac myocytes and transgenic mice. Deacetylation-mediated activation of Nav1.5 by SIRT1, and its modulation by GPD1-L, will identify an entirely new mechanism for regulation of cardiac INa. In doing so, it will open the door for pharmacologic SIRT1 activators as potential therapeutic agents in patients at risk for cardiac arrhythmias dues to inherited and acquired disturbances in cardiac INa.